Process for evaporating and superheating thermally unstable hydrocarbon mixtures



3,330,760 ALLY July 11, 1967 I J. HIRSCHBECK ET PROCESS FOR EVAPOHATING AND SUPERHEATING THERM UNSTABLE HYDROCARBON MIXTURES Filed Oct. 12, 1965 komofiwwr N United States Patent 3,330,760 PROCESS FOR EVAPORATING AND SUPER- HEATING THERMALLY UNSTABLE HY- DROCARBON MIXTURES Josef Hirschheck and Josef Riedl, Burgkirchen (Alz), Germany, assignors to Farbwerke Hoechst Aktiengesellshaft vorrnals Meister Lucius & Bruning, Frankfurt am Main, Germany, a corporation of Germany Filed Oct. 12, 1965, Ser. No. 495,186 Claims priority, application Germany, Oct. 20, 1964, F 44,267 5 Claims. (Cl. 208-92) Cracking processes are carried out on an industrial scale in which, in addition to the thennostable hydrocarbons generally used as starting material, less thermostable reaction products that shall not be obtained but are suitable for being transformed into the final product are reconducted into the reaction zone. Less stable reaction products of this type are, for example, higher olefins which can be transformed into lower olefins, particularly ethylene. In general, cracking requires high temperatures and a high amount of energy for the endothermal reaction and, therefore, attempts are made to introduce the hydrocarbons to be reacted into the cracking zone at as high a temperature as possible.

It is known that hydrocarbons preponderantly containing saturated compounds, which are referred to in the following as thermally stable compounds, can be heated at temperatures up to above 400 C. without noticeable decomposition, but that olefin-containing hydrocarbon mixtures may be subject to undesirable modifications even at lower temperatures, such as polymerization or decomposition with carbonization. Moreover, polymers or hydrocarbons which reach the cracking zone in the non vaporized state readily decompose with carbonization. It has been found that these phenomena may endanger the operation of an industrial plant even when occurring to a very minor extent.

The hydrocarbons to be recycled in the aforesaid cracking processes contain practically always thermally unstable constituents which often do not allow the desired preheating.

The present invention provides a process for evaporating thermally unstable hydrocarbons and superheating them to temperatures above 350 C., which comprises evaporating the said hydrocarbons or mixtures thereof at a temperature below 180 C. by indirect heat transfer, freeing the vapors from liquid constituents and formed polymers, directly combining the vapors with a gas current of thermally stable hydrocarbons heated at 400 to 500 C., and conducting the combined gas current into the reaction zone. When proceeding in this manner, the aforesaid difiiculties are avoided. Simultaneously, in the cracking process the thermal efiiciency of the cracking unit is improved.

The thermally unstable hydrocarbons are preferably evaporated at a temperature that does not exceed 15 0 C. The gas current obtained after having combined the thermally unstable and the thermally stable gases generally has a temperature in the range of from 350 to 400 C.

It is advantageous to add steam to the thermally unstable hydrocarbons to be evaporated in an amount of 5 to by weight.

The process according to the invention is particularly suitable for cracking hydrocarbons. In this case, the gas current of thermally stable hydrocarbons heated at 400 to 500 C. consists of the hydrocarbon mixtures used as starting material, generally a crude oil fraction. The thermally unstable hydrocarbons to be evaporated and further heated are preferably the unsaturated hydrocar- 3,330,760 Patented July 11, 1967 bon mixtures obtained in cracking processes and to be reconducted into the reaction zone for being subjected to further cracking reactions. It is not absolutely necessary, however, to use by-products of the cracking process. The process according to the invention can also be carried out with advantage with thermally unstable hydrocarbons or mixtures thereof of other origin.

The thermally unstable hydrocarbons or hydrocarbon mixtures to be used according to the invention generally consist of higher monoolefins or polyolefins mostly having 3 to 8 carbon atoms, which may contain small amounts of the corresponding acetylenes. By the stepwise heating according to the invention, above all the internal further heating of the thermally unstable hydrocarbons in the second stage, superheating is avoided which would forcibly occur with external heating and involve an undesired and uncontrollable transformation of the reactants before they reach the reaction zone.

An essential characteristic of the process according to the invention is the separation of liquid constituents and formed polymers from the thermally unstable hydrocarbons or mixtures thereof in the course of the first heating and before they are combined with the thermally stable hydrocarbons. This can be done in separators of known construction. A further characteristic of the process according to the invention is the dilution, performed simultaneously with the second heating, of the thermally unstable hydrocarbons with thermally stable hydrocarbons and the resulting diminution of the partial pressure of the former hydrocarbons, whereby further polymerization reactions are avoided and liquid particles which may be entrained from the separator are again evaporated.

The accompanying drawing diagrammatically illustrates by way of example a device for carrying out the process of the invention.

Steam serving as heat carrier for the cracking reaction is heated in 1 at a temperature of 1300 to 1500" C., partially by indirect and partially by direct superheating, and conducted into reactor 2. Unsaturated hydrocarbons which are obtained in gas separation 4 and not desired as final products are evaporated in evaporator 5 by indirect heat transfer, separated from non volatile constituents in separator 6, immediately combined with freshly used saturated hydrocarbons evaporated in evaporator 7 and superheated at 400500 C. in heater 8 and the combined gas current is conducted into reactor 2. The heat set free in cooling the reaction products is utilized in heat exchanger 3 for the production of steam.

The following example serves to illustrate the invention but it is not intended to limit it thereto.

EXAMPLE 4,650 kilograms/hour of saturated hydrocarbons (gasoline mixture) were heated at 450 C. 2,000 kilograms/hour of a mixture of unsaturated hydrocarbons consisting of olefins and dienes with 3 to 8 carbon atoms were heated at C., freed in a separator from formed liquid constituents and immediately combined with the stable hydrocarbons heated at 450 C. The combined gas currents having a temperature of 360 C. were then introduced into the reactor. The reactor was operated in this manner over 5 months without an interruption in operation owing to deposits being necessary.

When the reflux was not treated as described above, the operation time of the reactor was considerably shorter.

We claim:

1. A process for evaporating thermally unstable hydrocarbons and superheating the said hydrocarbons at a temperature above 350 C., which comprises (a) evaporating at least one thermally unstable hydrocarbon by indirect heat transfer at a temperature below 180 C., (b) freeing the vapors obtained in (a) from liquid constituents and formed polymers, (c) immediately combining the vapors obtained in (b) with a gas current of thermally stable hydrocarbons heated at 400 to 500 C. and (d) conducting the combined gas current to a reaction zone.

2. The process of claim 1, wherein the thermally unstable hydrocarbons are evaporated at a temperature below 150 C.

3. The process of claim 1, wherein 5 to 10% by weight of steam are added to the thermally unstable hydrocarbons to be evaporated.

4. The process of claim 1, wherein the unstable hydrocarbons are unsaturated hydrocarbon mixtures originating from cracking reactions the combined gas current is conducted a reaction zone for further cracking.

5. The process of claim 1, wherein the gas current consisting of thermally stable hydrocarbons is a hydrocarbon mixture to be subjected to cracking.

References Cited UNITED STATES PATENTS DELBERT E. GANTZ, Primary Examiner.

H. LEVINE, Assistant Examiner. 

1. A PROCESS FOR EVAPORATING THERMALLY UNSTABLE HYDROCARBONS AND SUPERHEATING THE SAID HYDROCARBONS AT A TEMPERATURE ABOVE 350*C., WHICH COMPRISES (A) EVAPORATING AT LEAST ONE THERMALLY UNSTABLE HYDROCARBON BY INDIRECT HEAT TRANSFER AT A TEMPERATURE BELOW 180*C., (B) FREEING THE VAPORS OBTAINED IN (A) FROM LIQUID CONSTITUENTS AND FORMED POLYMERS, (C) IMMEDIATELY COMBINING THE VAPORS OBTAINED IN (B) WITH A GAS CURRENT OF THERMALLY STABLE HYDROCARBONS HEATED AT 400 TO 500*C. AND (D) CONDUCTING THE COMBINED GAS CURRENT TO A REACTION ZONE. 